The invention concerns a burner for gaseous fuels featuring low emissions of NOx and CO, particularly a burner fed with a hyper-stoichiometric mixture of fuel and air, that is with a quantity of primary air exceeding the stoichiometric value. The hyper-stoichiometric mixture makes it possible to reduce the temperature of the flames thus reducing the quantity of NOx formed during combustion.
Burners fed with a hyper-stoichiometric mixture involve the problem of flame stability because the flame tends to be extinguished intermittently and, as a consequence, to get detached from the surface of the burner, which causes an unsatisfactory operation of the burner, emission of unburned substances and high quantity of CO in the fumes produced by the combustion. If the ratio between air and fuel in the air-fuel mixture increases the instability of the flame increases as well. This problem may be made worse by the presence of air or nitrogen in the gaseous fuel feeding the burner and by pressure and moisture conditions of the environment wherefrom air, which is mixed with the gas fuel, is taken.
According to the state of art, one has tried to solve this problem by distributing discharge openings for the air-fuel mixture on the burner diffuser. The discharge openings are shaped as slots and are arranged in continuous or non-continuous rows separated from each other by a diffuser portion devoid of slots. A vacuum and a temperature higher than the temperature of the other parts of the diffuser are generated during combustion in the diffuser portion, which oppose flame extinction and consequent detachment of the flame from the surface of the burner.
However, the secondary air which laps the flames tends to cool them and to dilute further the mixture of air and gas-fuel flowing through the slots. Consequently, instability of the flame may occur when the aeration rate is high and a reduction of the content of NOx in the combustion products over a certain limit can not be achieved.
It is an object of the present invention to obtain a burner having a very low content of NOx in the combustion products and an improved stability of the flames.
The invention solves the above mentioned technical problem by adopting a burner fed with a hyper-stoichiometric mixture of air and gas-fuel, comprising a diffuser on which rows of first slots for a flow of said mixture are provided. The slots of each row are similarly shaped, evenly spaced and parallel to each other. The rows are parallel to each other and arranged on either sides of an intermediate portion of the diffuser having a substantially constant width; the intermediate portion being provided with further slots for the flow of the mixture that are non parallel to the first slots.
The further slots may be arranged in one or more continuous or non-continuous rows parallel to the median line of the intermediate portion of diffuser. The further slots may be arranged parallel to, or even inclined with respect to, the median line of the intermediate portion of diffuser, and may be rectilinear or curvilinear.
The burner of the invention has the advantage that auxiliary flames are generated in the intermediate portion of diffuser, between the main flames generated by two adjacent rows of first slots. The auxiliary flames heat those portions of the main flames facing towards the intermediate portion, thus preventing extinction of the main flames and their instability.
In an advantageous embodiment of the present invention, the further slots are so shaped that the speed of the mixture of air and gas-fuel flowing through them is lower than the speed of the mixture flowing through the first slots. This embodiment has the advantage that very stable auxiliary flames are generated, which act as anchor means for the main flames.
In a further advantageous embodiment of the present invention, the rate of flow of the mixture of air and gas-fuel flowing through the further slots of the intermediate portion of diffuser is lower than the rate of flow of the mixture flowing through the first slots. This result may be obtained by means of a whole surface of the further openings being substantially lower than the whole surface of the first slots, or by so shaping the further slots that the mixture is subject to heavy energy losses flowing through them. This embodiment has the advantage that the mixture of air and gas-fuel flowing through the further slots of the intermediate portion of diffuser does not affect significantly the structure of the flame formed on the rows of first slots.
In a further advantageous embodiment of the present invention the intermediate portion of diffuser comprises a heat-resistant material which may reach, without being damaged, a temperature higher than the remaining portions of the diffuser. This embodiment has the advantage that the mixture of air and gas-fuel flowing through the further slots is heated by the heat stored in the intermediate portion of the diffuser, which increases the combustion speed of the mixture allowing extremely stable auxiliary flames to be obtained that act as anchor means for the main flames.
In a further advantageous embodiment of the present invention, the heat-resistant material has a porous or spongy structure, which allows the mixture of air and gas-fuel to pass through it. The material having a porous or spongy structure may be a ceramic material. This embodiment has the advantage that the mixture of air and gas-fuel flowing through the further slots is heated and its speed is reduced at the same time.
In a further advantageous embodiment, the material having a porous or spongy structure is accommodated in a housing provided in the intermediate portion of the diffuser. This embodiment has the advantage of replacing the material quickly and easily, if necessary.
In a further advantageous embodiment, the heat-resistant material has a reticular structure. This embodiment has the advantage that the thermal deformations to which the material is subjected do not cause transmission of considerable mechanical stresses to the other parts of the burner.